You can go to the Arma 3 settings, and under the betas tab. There is a beta access box where you can type in "Arma3Legacy174" and it'll unlock the previous version for you to use. I think it is only available until the next update.

1) Right click Arma 3 in the steam window
2) Select properties and a new window will open
3) Click the 'Local Files' tab
4) Select 'Verify Integrity of Game Files'.
Steam should detect the missing files and download them.

Assuming that blackpixxel is okay with people doing so, all one has to do is open the pbo, remove the mission folder, remove any reference to the mission from the config.bin , then repbo the file. It is an easy fix that would only take 5 minutes.

Yes, but does it not have the mission pbo? Then all you would need to to is extract the pbo to your Documents\Arma 3\missions (or anywhere if you're not planning on editing it with the eden editor), and copy the scripts into that folder and make adjustments, then re-pbo the file and drop it into your Arma 3\Missions or Arma 3\MPMissions where it'll show up under scenarios.